Filters for water filtration and, in particular, waste water filtration, typically comprise woven structures where filaments are precisely spaced in the warp and weft (i.e., lengthwise/machine direction and crosswise/widthwise direction), forming essentially a single layer sieve with a precise pore geometry. These structures are typically made from filaments that are at least 20 to 100 microns or more in diameter, depending on the degree of openness required and the final pore size desired. The pore size of such structures is controlled by the spacing of the filaments in the machine and cross directions. Due to the uniformity of the construction of these types of filters, the filters are rated based upon the size of the fixed pores. Such materials act as surface filters, retaining particulates on the surface of the filter and not deep within the structure. Particles larger than the pore size are retained on the surface of the filter and particles smaller than the pore size pass through. For example, a “20 micron filter” will retain particles that are 20 microns or larger in diameter, while smaller particles will pass through. Common filters are “20 micron filters,” “10 micron filters,” “5 micron filters,” and “1 micron filters,” capable of retaining particles having diameters of 20 microns or more, 10 microns or more, 5 microns or more, and 1 micron or more, respectively.
As these woven structures are surface filters, the pores often become fouled with retained particulates during use. When a surface filter has become fouled, the surface must be cleaned to remove the collected particulates and render the filter ready for reuse. For example, the surface filter can be backwashed using water jet spays to release the retained particulates. Sometimes, the backwashing is followed by a chemical wash to remove deposition of other matter on the surface of the woven filter, such as proteins and other substances present in the liquid stream that are deposited on the filter surface.
In contrast to woven surface filters, nonwoven fabrics typically do not have precise pore geometries and sizes and commonly comprise a range of pore capillaries throughout the structure. The pores of a nonwoven material are not planar and create a tortuous path within the material that can allow particulates to be effectively retained within the structure (via depth, rather than surface, filtration). Accordingly, depth filters generally provide for longer term use, as depth filters often can retain larger quantities of particulate matter than surface filters. However, the tortuous paths within nonwoven materials, and the usual lack of dimensional stability of nonwoven materials, render it difficult to remove retained particulates captured within nonwoven depth filters. Nonwoven filters are thus typically used in applications where the pore size requirements are larger, such as in swimming pool and spa filters. Generally, such filters are replaced upon fouling and/or reduction in capture efficiency, rather than being cleaned for reuse.
It would be desirable to provide a filter material combining the advantages of surface filtration (e.g., ease of cleaning and dimensional strength) with the advantages of depth filtration (e.g., longer-term use).